Document CONAN_SYSTEM_INCLUDES

[ArekPiekarz]

In CMake using set(CONAN_SYSTEM_INCLUDES ON) makes Conan add library headers as "system headers". Thanks to that the compiler doesn't check for warnings in them.

I only learned about that variable through conan-io/conan#2629.

Note that I don't know if it is usable in other build systems, but I think it may be.

[darcamo]

That would be very useful.

I spent a lot of time searching how to suppress the warnings from the libraries installed by conan that could be saved if CONAN_SYSTEM_INCLUDES was in the documentation.

[johnmcfarlane]

+1. Surprised that ON isn't the default.

[voidbar]

+1. I only reached this after ctrl+f'ing through the conanbuildinfo.cmake. Would be very useful to add some kind of documentation.